1) Long sessions are more suitable on a laptop/desktop than on a smartphone (the average session time is ~15mn).
2) There is still a nerdy userbase (mostly early users) who prefer to use the website with all the features.
3) The app is relatively recent (April 2016 vs 2012 for Facebook).
4) The official app is not really good IMO (e.g. scrolling is laggy and the UI could be better).